docs: correct VK invite-link reality + note desktop board drag-to-pan

Two doc edits that did not land alongside their code:

- FUNCTIONAL (+_ru) claimed a friend invitation is shareable as a Telegram
  *or VK* deep link "that opens it directly". On VK that is false: vkShareLink
  emits a plain vk.com/app<id> link (VK forwards no payload to the Mini App),
  so the recipient enters the copied code by hand (#142, commit d76f1f4 pivoted
  off the earlier #f<code> plan; the doc kept the old wording).
- UI_DESIGN omitted the desktop / landscape-iframe mouse drag-to-pan of the
  zoomed board added in #141 (touch scrolls the viewport natively; a mouse
  cannot, so an explicit drag-to-pan handler moves it).

docs/FUNCTIONAL.md

@@ -125,7 +125,8 @@ and any incidental perpendicular words are ignored and not scored — while on i
 standard Scrabble. English games are always standard and show no such toggle. In auto-match
 the choice joins the pairing key, so a player only meets opponents who picked the same rule. Friend games (2–4) are
 formed by inviting players from the friend list (an invitation, like a friend code,
-is shareable as a Telegram or VK deep link that opens it directly): the inviter chooses the
+is shareable as a Telegram deep link that opens it directly — on VK, which forwards no payload to the
+Mini App, the link only opens the app and the recipient enters the copied code by hand): the inviter chooses the
 settings and the game starts once every invitee has accepted — any decline cancels it, and an unanswered invitation
 expires after seven days.